Herpes simplex virus type 1 (HSV-1) is a linear double stranded DNA virus with genome size of about 152 kb. The genome of HSV-1 is encapsided by an icosadeltarhedral capsid surrounded by a viral envelope. HSV replicates in epithelial cells and establishes life-long latent infection in neuronal cell bodies within the sensory ganglia of infected individuals. The latent viral genome is maintained in an episomal state and does not ordinarily cause serious disease or interfere with normal cellular function (Rock, et al., J. Virol. 55:849-852 (1985)). These characteristics have made HSV of particular interest for use as a vehicle for gene therapy procedures designed to treat diseases of the CNS (Latchman, Curr. Gene Ther. 2:415-426 (2002); Glorioso, et al, J. Neurovirol. 9:165-172 (2003); Jacobs, et al., Neoplasia 1:402-416 (1999); Advani, et al., Clin. Microbiol. Infect. 8:551-563 (2002); Martuza, et al., Science 252:854-856 (1991)). One difficulty that has been associated with the development of such procedures has been in finding vectors that induce high expression levels of delivered genes and do so in a manner that can be tightly regulated.
During the past decade, significant progress has been made in developing genetic switches that can be used to control the expression of recombinantly delivered genes (Clackson, Gene Ther. 7:120-125 (2000); Gossen, et al., Proc. Nat'l Acad. Sci. USA 89:5547-5551 (1992); Gossen, et al., Science 268:1766-1769 (1995); No, Proc. Nat'l Acad. Sci USA 93:3346-3351 (1996); Wang, et al., Proc. Nat'l Acad. Sci. USA 91:8180-8184 (1994); Rivera, et al., Nat. Med. 2:1028-1032 (1996)). In the case of prokaryotic elements associated with the tetracycline (tet) operon, systems have been developed in which the tet repressor protein is fused with polypeptides known to modulate transcription in mammalian cells. The fusion protein has then been directed to specific sites by the positioning of the tet operator sequence. For example, the tet repressor has been fused to the activation domain of transactivator (VP-16) and targeted to tet operator sequences positioned upstream from the TATA element of promoter of a selected gene (Gossen, et al., Proc. Natl. Acad. Sci. USA 89:5547-5551 (1992); Kim, et al., J. Virol. 69:2565-2573 (1995); Hennighausen, et al., J. Cell. Biochem. 59:463-473 (1995)). The tet repressor portion of the fusion protein binds to the operator thereby transporting the VP-16 activator to the specific site where the induction of transcription is desired. An alternative approach has been to fuse the tet repressor to the KRAB repressor domain and target this protein to an operator placed several hundred base pairs upstream of a gene. Using this system, it has been found that the chimeric protein, but not the tet repressor alone, is capable of producing a 10 to 15-fold suppression of CMV regulated gene expression (Deuschele, et al., Mol. Cell Biol. 15:1907-1914 (1995)). One problem with these types of systems is that a portion of fusion proteins corresponding to the mammalian transactivator or repressor trends to interact with cellular transcription factors and cause pleiotropic effects.
Recently, a tetracycline-inducible transcription switch for use in mammalian cells was developed (U.S. Pat. No. 6,444,871; Yao, et al., Hum. Gene Ther. 9:1939-1950 (1998)). This system was highly successful at regulating gene expression and has been used in developing plasmid-based vectors that are now sold commercially (T-REx™, Invitrogen, CA).